It is known that rubber compositions, such as, typically, tyre tread rubber compositions, comprise particulate reinforcing fillers. Common reinforcing fillers include carbon black and silica; other known fillers include, for example, clays, calcium carbonates, cellulose fibres and starch. Carbon black and silica are generally used in relatively large amounts in rubber compositions to impart a desirable level of reinforcement to the rubber.
In recent years, silica has become the filler of choice for tyre tread compositions (see, for example, Okel, T. A. et al, “Advances in Precipitated Silicas for Passenger and Truck Tyre Treads”, Progress in Rubber and Plastics Technology, Vol. 15, No. 1, 1999, the disclosure of which is incorporated herein by reference). Many conventional tyre tread compositions now incorporate silica, either as a sole reinforcing filler or as a main filler mixed with smaller amounts of one or more other fillers. The silica is generally present in a relatively high amount, typically up to 150 parts by weight based on 100 parts by weight of the rubber.
However, silica-filled rubber tyre tread compositions suffer from certain substantial disadvantages. Precipitated silica is expensive in comparison with carbon black, and causes difficulties in processing and manufacture of rubber compositions incorporating it. A precipitated silica filler requires complicated high temperature processing when formulating the rubber composition. There can be a tendency for components of the curing system of the rubber composition and/or organic non-rubber impurities of the rubber to adsorb onto the silica particles, leading to a reduction in the cross-link density below theoretically achievable values, and poor dispersion of the filler into the rubber compositions, which in turn can lead to undesirable properties of the tyre tread product.
Silica fillers give rubbers which generally exhibit a reduced rolling road resistance (i.e. a reduced tendency for kinetic energy of the vehicle to be lost as work of deformation of the tyre) in comparison with rubbers incorporating other filler systems. On the other hand, unless an expensive coupling agent such as an organosilane is also present, this desirable reduction in rolling resistance is typically accompanied by an undesirable reduction in wet traction and/or in resistance to abrasion.
Moreover, it is found in practice that the organosilane cannot effectively be pre-coated onto the silica particles, but must generally be handled separately and included into the mix at the time of compounding the rubber composition. The manufacture and transportation of the organosilane involves an appreciable energy cost. In terms of overall “greenness” or energy efficiency, the gains of reduced fuel consumption and enhanced tyre life and performance found with silica-filled tyre treads are offset by substantial energy costs of this type.
Natural rubber is a convenient material for tread portions of radial truck tyres, in view of its excellent uncured strength and tackiness and reduced tendency to cut and chip, in comparison with synthetic rubbers. However, it has been reported that silica—even with a bonding agent present—is not compatible with natural rubber, due in part to the adsorption of organics such as proteins and fatty acids from the natural rubber onto the surface of the silica particles, leading to problems of poor abrasion resistance. Attempts to increase the proportion of organosilane bonding agent result in a reduction in the tear resistance of the rubber composition (see, for example Freund, B., Eur. Rubber J. 180(8), p. 34, 1998, the disclosure of which is incorporated herein by reference).
It is desirable to be able to use a lower amount of silica in rubber compositions without a corresponding reduction in reinforcement of the rubber and without any loss of rubber properties.
Carbon black has long been known as a particulate filler for tyre rubber compositions. However, it generally cannot be used in applications where the rubber composition needs to be pigmented. In addition, the carbon black must have a very fine particle size, leading to increased financial and energy costs in the manufacture. Carbon black can also cause an undesirable build-up of frictional heat in a tyre tread rubber composition.
Investigations have been conducted into multi-phase particulate fillers, which seek to achieve a balance of parameters which a single filler material alone cannot achieve.
For example, apparently encouraging results have been reported for the use of carbon-silica dual phase fillers (CSDPF) in natural rubber treads for truck tyres, potentially reducing the amount of the organosilane bonding agent required (see, for example, Wang, M-J et al, “Carbon-Silica Dual Phase Filler, A New Generation Reinforcing Agent for Rubber: Application to Truck Tire Tread Compound” in Rubber Chemistry and Technology, Vol. 74, 2001, pp. 124-137, the disclosure of which is incorporated herein by reference).
There has been a general appreciation that at least partial replacement of particulate silica and/or carbon black by particulate kaolin can offer advantages in the manufacture of tyre tread rubber compositions (see, for example, U.S. Pat. No. 5,591,794, U.S. Pat. No. 5,840,795, and U.S. Pat. No. 5,871,846; EP-A-0170606, EP-A-0655480, EP-A-0678549, EP-A-0732362, and EP-A-0894819; as well as the publications cited therein and in the results of the relevant official searches). GB-A-2067535 describes the use of particulate kaolins to partially replace carbon black in tyre carcase rubber compositions. The disclosures of all these documents and prior publications are incorporated herein by reference.
However, there are substantial practical difficulties in the use of particulate kaolin as a filler for rubber compositions, and no proposal has been completely satisfactory for modern requirements. Particular difficulties are encountered with rubber compositions for tyre treads, which generally require a particular balance of properties such as a low rolling road resistance, a high degree of abrasion resistance and wet traction, and a high tensile strength and modulus of elongation. Conventional particulate kaolin tends to impair the properties of the rubber compositions and the performance of the tyres, in comparison with compositions filled with precipitated silica and/or carbon black in the absence of particulate kaolin, and generally the sought-after reduction in the required amount of the coupling agent has not been achievable.
There is therefore a need for a particulate kaolin suitable for use as a filler for rubber compositions, for example tyre tread rubber compositions, which can at least partially replace silica and/or carbon black while maintaining acceptable physical properties of the rubber compositions and acceptable performance characteristics of the tyres.
The present invention seeks to go at least some way towards answering this need, or at least to provide an acceptable alternative.
We have now developed an improved kaolin clay product which, when used as a filler or extender material in a rubber composition, provides a composition having an improved ease of manufacture and enhanced properties.